The invention relates to actively controlled structures and in particular to active control of buckling in beams or columns loaded in compression.
For many physical geometries, buckling is a factor limiting the maximum compressive force that may safely be applied to a member. Indeed, for many long slender members, the strength limitation imposed by buckling is several orders of magnitude more important than other factors limiting the loading of the member, such as plastic deformation.
Previous work in active control as it relates to beams and columns has included vibration control for application to large space structures. In particular, one group at MIT has done much work involving the use of piezoelectric actuators to damp out various vibration modes.
Other work has been done at Catholic University in Washington, D.C. As an axial load on a column is increased, bending begins. At about a quarter of the buckling load, this bending becomes quite noticeable. One part of the Catholic University work involves sensing when this bending begins and using Nitinol actuators to reduce the load on the column, thereby preventing the onset of significant bending and preventing buckling of the column. This work appears to make complex structures more robust by shifting weight to other supporting members when one member becomes overloaded.
Another aspect of this work involves the use of Nitinol shape memory wires, embedded within a beam, to control the beam's curvature. This work seems to be aimed at adaptive structure applications, in which it is desirable for a single beam to take on different shapes during different stages in the construction process. For instance, when constructing a long bridge out of smaller segments, actuators can arrange for the bridge segments to arch upwards, both to correct for differences in height between the two land masses being joined by the bridge, and to correct for bending caused by heavy loads crossing the bridge itself.
This nitinol wire approach has also been used to forcibly correct the bending that arises when a beam is axially loaded. It allows the beam to bend as the load approaches (but does not exceed) the buckling mode, and then uses the nitinol wires to stiffen the beam (on a time scale of 3-4 seconds) such that it takes on the desired shape.